feat(rust): add support for deep object query parameters

[iamnamananand996]

Description

Refs: Related to PHP implementation in #11765

Adds deep object query parameter encoding support to the Rust SDK generator. This enables nested objects to be serialized using bracket notation (e.g., filter[name]=john&filter[age]=30) following the OpenAPI 3.0 deepObject serialization style.

Link to Devin run: https://app.devin.ai/sessions/64b3914c128f4cfa957eaab189b9c426
Requested by: @iamnamananand996

Changes Made

• Added serialize_deep_object<T> method to QueryBuilder for encoding nested objects with bracket notation
• Added serialize_deep_object_array<T> method for handling arrays of nested objects
• Added flatten_json_value helper function that recursively flattens JSON values into query parameter format
• Added json_value_to_string helper for converting JSON primitives to strings
• Added comprehensive unit tests covering:
  • Simple nested objects
  • Deeply nested objects
  • Arrays (exploded format)
  • Arrays of objects
  • Mixed nested objects with arrays
  • None/null value handling

Testing

• Unit tests added/updated
• Manual testing completed

Human Review Checklist

• Verify flatten_json_value correctly handles recursive object/array structures
• Confirm the bracket notation format matches OpenAPI deepObject serialization spec
• Note: This PR adds the capability to QueryBuilder but does not update the generator to automatically use these methods - that would be a follow-up change

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